The lightest structural metal is magnesium and although it's pretty soft the addition of a few trace elements makes it strong enough to turn into cameras, laptop frames and even into car engines for VW Beetles back in the 50's and 60's. Unfortunately, however tempting it may be to use magnesium as a replacement of steel, which is over three times as heavy, there are some considerable disadvantages. First, even when doped with those strengthening trace elements magnesium remains a metal too soft to withstand much pressure or high temperatures and, worse, it can catch a fire that burns white hot which becomes downright explosive if you try to put it out with water. Them's some scary properties.

While magnesium is fairly cheap even in 99.95% purity like this cube it is difficult to cut and polish increasing its final cost considerably. However, having one with which to weigh off against a cube of tungsten makes for a very interesting comparison!

Weight for the 10mm cube is 1.74±0.05g and 28.5±0.2g for the 25.4mm